Curable silicone compositions are used as sealants, fillers or coatings for electric or electronic parts. On curing, they form silicone gels, silicone rubbers, or silicone resins of superior electrical characteristics, heat resistance, weatherability, and the like. For example, Japanese Patent JP-C-48-017847 discloses a curable silicone composition comprising a diorcanopolysiloxane with silicon bonded vinyl groups at the molecular chain terminals and in the side chains of the molecular chain, a diorganopolysiloxane having silicon-bonded hydrogen atoms at the molecular chain terminals or in the side chains of the molecular chain, or both, and a platinum-based catalyst.
U.S. Pat. No. 4,374,867 to Brown, et al. discloses a curable silicone composition comprising a branched dimethylpolysiloxane having silicon-bonded vinyl groups at the terminal ends of the molecular chain, an organopolysiloxane having silicon-bonded hydrogen atoms, and a platinum-based catalyst.
Japanese Patent Application JP-A-05-209127 discloses a curable silicone composition comprising a diorganopolysiloxane having two or more silicon-bonded alkenyl groups in each molecule, with a cyclic diorganosiloxane content of not more than 0.5 wt %, an organopolysiloxane resin having silicon-bonded alkenyl groups, a diorganopolysiloxane having silicon-bonded hydrogen atoms at both terminal ends of the molecular chain, and a platinum-based catalyst.
Diamine compounds have been suggested for improving the storage stability of such curable silicone compositions in U.S. Pat. No. 4,801,642 and 4,791,186 to Buentello et al. and U.S. Pat. No. 5,334,687 to Ikeno.
However, when these curable silicone compositions were cured, the cured products had insufficient flame resistance. This problem was particularly conspicuous in silicone gels. For this reason, it was impossible to seal or fill electric or electronic parts exposed to elevated temperatures over an extended period of time using these curable silicone compositions, especially silicone gel compositions.
Various inorganic flame resistance-imparting agents have been compounded with curable silicone compositions to improve flame resistance. For example, Japanese Patent JP-C-61-069865 discloses a curable silicone composition comprising an organopolysiloxane having two or more silicon-bonded alkenyl groups in each molecule, an organopolysiloxane having two or more silicon-bonded hydrogen atoms in each molecule, a reinforcing silica powder, an aluminum hydroxide powder, a clayey mineral powder, a triazole compound, and a platinum-based catalyst.
Japanese Patent No. JP-C-06-016937 discloses a curable silicone composition comprising an organopolysiloxane having, on the average, 0.1 to about 2.0 silicon-bonded vinyl groups in each molecule; an organopolysiloxane having, on the average, more than two silicon-bonded hydrogen atoms in each molecule; an inorganic filler selected from the group consisting of ground quartz, metallic carbonate, and carbon black; and a platinum-based catalyst.
However, sufficient flame resistance could not be obtained without compounding large quantities of inorganic flame resistance-imparting agents in these curable silicone compositions. As a result, the curable silicone compositions had insufficient flowability and handling properties, the external appearance of the resultant silicone cured products was non-transparent and was limited to white, black, or gray colors. Furthermore, when curable silicone compositions containing the inorganic flame resistance-imparting agents are stored, the flame retardants can precipitate.
The object of the present invention is to produce a curable silicone composition that forms a silicone cured product with superior flame resistance.